(1) Field of the Invention
This invention relates to a variable equalizer. More particularly, it relates to a circuit which compensates for the fluctuations of the frequency characteristics of transmission media such as coaxial cable and pair cable, the fluctuations being dependent upon or ascribable to the lengths of the transmission media and other factors.
(2) Description of the Prior Art
As the variable equalizer of this type, there has heretofore been used the so-called Bode type variable equalizer with which the arrangement of a circuit device is simple. The transfer function of the Bode type variable equalizer is expressed by: ##EQU1## Here, x denotes a variable value which is independent of the frequency, and Y a function which has a predetermined frequency characteristic. In order to realize with an integrated circuit the variable equalizer which has such a transfer function, a feedback circuit is required. Therefore, the variable equalizer is not suitable as a circuit of high operating speed. Recently, owing to the progress of a communication system using optical fibers, variable equalizers are often operated in a very broad band. Especially for the equalizer requiring such a broad-band and high-speed operation, the Bode type variable equalizer in the prior art is not appropriate.
A variable equalizer which employs no feedback circuit has been known (U.S. Pat. No. 3,652,952). The concrete circuit arrangement thereof, however, requires circuits whose variation characteristics differ from each other and have a predetermined relation therebetween. For the reasons that the realization of the circuits at high precision is difficult and that the versatility in the circuit design is limited, it is difficult in principle to reduce compensation errors.